Proppants and proppant additives are increasingly used in screenless completions. In these applications, no screen or annular gravel pack is used to support the proppant in the perforation and the fracture. The proppant pack should not flow back in the bore hole if the stimulation treatment is successful. For screenless completions to be successful for the long term, the proppant pack and perforation tunnel must retain stability and conductivity under production conditions of temperature, fluid flow, stress cycling, and drawdown pressure during the life of the well. Therefore, screenless completions necessitate that the CRCP attain the maximum possible strength in the fracture and in the perforation tunnels. The strength is necessary to prevent proppant flowback anticipated at high production rates following fracturing. The practice in the prior art has been to evaluate proppants by measuring either consolidation strengths or fracture conductivity, the tests being conducted under simulated downhole conditions with an API cell.
Proppant hydraulic fracturing is a part of a treatment performed to stimulate oil/gas wells to enhance production, and in sandstone reservoirs it serves the purpose of mitigating production of sand due to the increased draw-down pressure. A CRCP is usually used at a final stage to prevent proppant flow-back upon putting the well on production.
The use of CRCP is intended to solve the problem of proppant flowback by having the curing resin form a pack that maintains its structural integrity when hydrocarbon production is commenced. It was known that the well should be closed and the fracture closed on the proppant to allow the resin to bind the proppant grains together in order to form a strong proppant bed or pack before a given well was put on production. The production engineers want to put the well on production as soon as possible, since the costs in time, labor, materials and equipment are substantial. However, it has been found that CRCP will flow back into the well when the well is put back on production.
No basis exists in the art for determining the required shut-in time, other than the time needed for a fracturing gel to break. Similarly, no consideration was given to the strength development of CRCP. Gel breakers are used in fracturing fluids to trigger gel degradation of polymeric materials predetermined period of the completion of a stimulation treatment. The shut-in time designed for fracturing treatments is based on the shut-in time required to achieve polymer degradation. There is no indication in the literature on how long it takes the CRCP to achieve its maximum strength and what property might be relied upon to determine its strength development. The failure to achieve a complete cure for the CRCP is counter-productive.
When a reservoir rock formation is fractured and proppants are pumped into the formation to maintain the opened flow paths following relief of the pressure of the fracturing fluid, the temperature of the reservoir in the fractured zone is altered, i.e., lowered, by introduction of the various fluids. Thus, it is known that the reservoir temperature decreases due to the cooling effect caused by injecting a large volume of fracturing gel that is at ambient surface temperature into the formation. However, this effect has not been considered when determining the in situ curing time of a given CRCP.
During the shut-in time, i.e., the time that the well is out of production, the temperature of the fluids and CRCP in the fractured zone increases as the introduced materials absorb heat conducted from the surrounding formation. This downhole temperature recovery over time can be measured and expressed graphically, i.e., by a plot or curve, or in a tabular form and stored in electronically.
The temperature recovery curve is characteristic for a given type of reservoir formation and is reasonably predictable or consistent for a given oil field or geological region, and depth. As will be understood by those familiar with the art, downhole temperature also varies with depth, the temperature generally being higher at greater depths.
A variety of resin products and CRCP are available from commercial sources. Test data is provided by the manufacturer that indicates the time required for complete curing and compressive strength development of the resin at a given constant temperature. In general, there is not a linear relationship between cure time and temperature, so that determination of the cure time for a batch of CRCP under conditions of changing temperature cannot be readily determined theoretically from uniform temperature and time data.
Currently, the duration of the shut-in time following a hydraulic fracturing treatment that uses CRCP to prevent proppant flow-back into the well with produced hydrocarbons does not account for the effect of shut-in time required for complete compressive strength development. As a result, proppant particles that have not completely cured to form a monolithic pack are displaced by the subsequently produced hydrocarbon and the value and expense of the treatment has been lost, at least in part.
The testing methods currently practiced in the industry to qualify proppant for field applications are based on the physical characterization of a number of parameters, such as specific gravity, absolute volume, solubility in HCl/HF acid, roundness, sphericity and bulk density. A sieve analysis, compressive strength and API crush tests are also performed. The API series RP 56, 58 and 60 are the principal procedures used to test conventional proppants for hydraulic fracturing treatments. At present however, there is no API testing procedure for CRCP proppants
It is therefore an object of the present invention to provide a new test method and associated apparatus set up for determining the minimum shut-in time after a CRCP has been introduced into the formation to effect complete curing of the resin and maximum pack strength under conditions that simulate actual reservoir conditions during and after fracturing treatment.
Another object of the present invention to provide a direct, reliable and easy to apply laboratory test method for qualifying a given CRCP for use in a reservoir under known stress and temperature conditions.
A further object of the invention is to provide a laboratory test method that is simple to apply and that produces reliable results for predicting time to achieve optimum compressive strength of a CRCP proppant pack under pressure and when the CRCP is subjected to a varying curing temperature that is representative of conditions in a subterranean treatment in which the proppant will be used.
Yet another object of this invention is to provide a laboratory test method for evaluating a number of different commercial CRCP products to develop a database of cure times under the same and different conditions to aid in the future selection of a CRCP product that will minimize the shut-in time, and thereby the costs associated with a fracturing treatment of a particular reservoir, under expected field conditions of pressure, temperature and temperature recovery.
A further object of this invention is to provide a laboratory test method that will prevent or minimize CRCP proppant degradation and the undesirable attendant flowback when a well is returned to production.
It is also an object of the invention to provide manufacturers and users of CRCP proppants with a laboratory test method for determining the effect of curing temperature variations on compressive strength development.